The maximum amount of fault current that a circuit breaker can safely interrupt without getting damaged is called the Interrupting Capacity of a Circuit Breaker. The current flowing through the circuit increases to extremely high levels when a short circuit occurs in an electrical system. The circuit breaker must be able to break this high fault current and stop the flow of electricity. If a circuit breaker cannot handle the fault current, it may fail or even explode.
In this technical guide, we will discuss everything about the interrupting capacity of circuit breakers. We will cover definitions, calculations, standards, selection guidelines, and practical examples.
1. What is Interrupting Capacity of a Circuit Breaker?
The interrupting capacity of a circuit breaker also known as breaking capacity or rupturing capacity is the maximum fault current that a circuit breaker can safely interrupt without sustaining damage. This value is expressed in amperes (A) or kiloamperes (kA).
For example, a circuit breaker might have a continuous current rating of 100 amperes. However, its interrupting capacity might be 10,000 amperes or 10 kA. This means the circuit breaker can safely interrupt fault currents up to 10,000 amperes. If the available fault current at the installation point exceeds 10,000 amperes, a different circuit breaker with higher interrupting capacity must be selected.
2. Why Interrupting Capacity Matters
Interrupting capacity is one of the most important ratings you need to consider when selecting circuit breakers. This rating directly affects the safety of your electrical installation and the people who work around it. A circuit breaker with the wrong interrupting capacity can fail during a fault. This failure can lead to equipment destruction, fires, injuries, and even fatalities.
Electrical faults happen unexpectedly in any system. A wire might break and touch another conductor. Insulation can fail due to age or damage. Someone might accidentally create a short circuit during maintenance work. When these faults occur, the current rises to levels many times higher than normal operating current.
Your circuit breaker must stop this fault current before it causes damage. For example, a 20-amp circuit normally carries 20 amperes or less. During a short circuit, that same circuit might try to carry 15,000 amperes or more. The circuit breaker needs enough interrupting capacity to handle this massive increase in current.
3. How Interrupting Capacity is Determined
The interrupting capacity depends on the construction and design of the circuit breaker. Different types of circuit breakers have different interrupting capacities. The method used to extinguish the arc between the contacts plays a major role in determining this capacity.
Air circuit breakers use air as the medium to cool and extinguish the arc. These breakers are suitable for lower interrupting capacities. Vacuum circuit breakers use vacuum as the arc-quenching medium and can handle higher fault currents. Oil circuit breakers use insulating oil to cool the arc. SF6 circuit breakers use sulfur hexafluoride gas, which has excellent arc-quenching properties and can handle very high interrupting capacities.
The contact material, contact speed, and arc chutes also affect the interrupting capacity. Faster contact separation means the arc has less time to build up. Better arc chutes direct the arc away from the contacts and help cool it down quickly. All these factors combine to determine how much fault current the breaker can safely interrupt.
4. Available Fault Current
Before selecting a circuit breaker, you need to calculate the available fault current at the installation point. This calculation requires knowledge of the system voltage, transformer capacity, and impedance values of all components in the circuit.
The fault current calculation starts at the power source. For a transformer, you can calculate the maximum fault current using the transformer rating and its impedance. For example, if you have a 1000 kVA transformer with 5% impedance connected to a 480-volt system, the maximum fault current at the secondary terminals would be approximately 24,000 amperes.
As you move farther from the source, the fault current decreases due to the impedance of cables, conductors, and other components. A point closer to the transformer will have higher available fault current than a point farther away. You must calculate the fault current at each location where you plan to install a circuit breaker.
5. Types of Interrupting Capacity Ratings
Circuit breakers have two main types of interrupting capacity ratings. These are the ultimate interrupting capacity and the service interrupting capacity.
5.1 Ultimate Interrupting Capacity (Icu)
The ultimate interrupting capacity is the maximum fault current that a circuit breaker can interrupt. After interrupting this level of fault current, the circuit breaker may not be suitable for continued use. It might require inspection, testing, or replacement before being placed back in service. The IEC standards use the symbol Icu for this rating.
5.2 Service Interrupting Capacity (Ics)
The service interrupting capacity is the fault current level that a circuit breaker can interrupt multiple times. After interrupting this level of fault current, the circuit breaker remains operational without requiring maintenance or replacement. The service interrupting capacity is usually expressed as a percentage of the ultimate interrupting capacity. Common values are 25%, 50%, 75%, or 100% of Icu.
6. Standards for Interrupting Capacity
Different regions use different standards for rating and testing circuit breaker interrupting capacity.
6.1 IEC Standards
The International Electrotechnical Commission publishes standards used throughout Europe, Asia, and many other parts of the world. IEC 60947-2 covers low voltage switchgear and controlgear including circuit breakers. This standard defines Icu and Ics ratings along with testing procedures.
6.2 ANSI/IEEE Standards
In North America, circuit breakers follow ANSI and IEEE standards. ANSI C37 series covers circuit breakers for various voltage levels. These standards use the term “interrupting rating” instead of “interrupting capacity.” The testing procedures differ somewhat from IEC standards.
6.3 UL Standards
Underwriters Laboratories provides standards for circuit breakers sold in the United States and Canada. UL 489 covers molded case circuit breakers. UL standards specify the interrupting rating as the maximum available fault current that the circuit breaker can safely interrupt.
7. How to Calculate Available Fault Current
Before selecting a circuit breaker, engineers must calculate the available fault current at the installation point. This calculation involves several steps.
Step 1: Determine the Source Impedance
The first step is to determine the impedance of all components between the power source and the point of installation. This includes the utility transformer, cables, busbars, and any upstream protective devices.
Step 2: Calculate the Fault Current
The available fault current can be calculated using Ohm’s law. Divide the system voltage by the total impedance to find the fault current. For three-phase systems, the formula is:
Fault Current = Voltage / (√3 × Total Impedance)
Step 3: Consider Motor Contribution
Motors connected to the system can contribute additional fault current during the first few cycles of a fault. This motor contribution must be added to the utility-supplied fault current for accurate calculations.
7.1 Example Calculation
Consider a 480V three-phase system with a 1000 kVA transformer having 5.75% impedance. The full load current of the transformer is approximately 1203 amperes. The available fault current at the transformer secondary can be estimated as:
Fault Current = Full Load Current / (Impedance ÷ 100)
Fault Current = 1203 / 0.0575 = 20,922 amperes
This means circuit breakers installed near this transformer must have an interrupting capacity of at least 22 kA to provide a safety margin.
8. How to Select the Right Interrupting Capacity
When selecting a circuit breaker, always choose one with an interrupting capacity that exceeds the available fault current. A safety margin of 10% to 20% above the calculated fault current is recommended.
8.1 Common Interrupting Capacity Ratings
Residential circuit breakers are available with interrupting capacities ranging from 10 kA to 22 kA. Commercial and industrial circuit breakers offer higher ratings from 25 kA to 200 kA. For installations close to large transformers or utility substations, very high interrupting capacity circuit breakers are required.
8.2 Series Rated Systems
In some cases, a downstream circuit breaker with lower interrupting capacity can be used if it is protected by an upstream device with higher capacity. This arrangement is called a series rated system. The upstream device limits the fault current that reaches the downstream device. Both devices must be tested and approved for use together in this configuration.
9. Testing Interrupting Capacity
Manufacturers put circuit breakers through intense testing to verify their interrupting capacity ratings. You cannot simply claim a breaker can handle 25 kA of fault current. The breaker must prove it can do the job under controlled laboratory conditions.
The test facility needs specialized laboratories with equipment capable of generating tens of thousands of amperes on demand. The test circuit connects the breaker to this power source along with instruments that record every detail of what happens during the fault interruption.
Testing standards require the breaker to interrupt multiple faults in sequence, not just one. The IEC standard uses an O-t-CO-t-CO sequence. The breaker opens to clear a fault, waits a bit, then closes back in and immediately opens again. After another pause, it does one more close-open cycle. This makes sense when you think about it. In the field, a breaker might need to handle multiple fault events. An automatic recloser could send the breaker right back into a fault condition.
10. Conclusion
Interrupting capacity is a fundamental specification that determines whether a circuit breaker can safely clear fault currents in your electrical system. Selecting the right interrupting capacity requires calculating the available fault current at each installation point and choosing breakers rated to handle that current or higher. This is not just a technical requirement but a safety necessity that protects people, equipment, and property.
11. Frequently Asked Questions (FAQs)
A: The circuit breaker may fail to clear the fault. The contacts can weld together, the housing can rupture, or a fire can start. This creates a serious safety hazard and can cause extensive equipment damage.
A: Yes, using a circuit breaker with higher interrupting capacity than required is perfectly acceptable. It provides an additional safety margin for your installation.
A: The interrupting capacity is marked on the circuit breaker nameplate or label. It is usually expressed in kA. You can also find this information in the manufacturer’s catalog or data sheet.
A: Yes, these terms are often used interchangeably. Interrupting capacity, interrupting rating, and short circuit rating all refer to the maximum fault current the circuit breaker can safely interrupt.
A: Symmetrical interrupting capacity refers to the steady-state fault current. Asymmetrical interrupting capacity includes the DC offset component that occurs in the first few cycles of a fault. ANSI standards use asymmetrical values while IEC standards use symmetrical values.